Conventional absorbent articles such as baby diapers, adult incontinence devices, and feminine napkins are typically made with a cellulose fiber fluff-based absorbent core sandwiched between a liquid pervious top sheet, which allows the unobstructed passage of fluid to the absorbent core, and a liquid impervious backing sheet usually of plastic material, which contains the absorbed fluid and prevents it from passing through the absorbent core and soiling the undergarments of the wearer of the absorbent article.
The absorbent core of these absorbent articles is typically constructed of defiberized wood pulp with or without superabsorbent polymer granules. The absorbent core is typically formed on a pad-forming unit of a converting machine on a carrier tissue to facilitate processing. Some absorbent core forming units are equipped with layering capability in which a second discrete fluff layer may be laid over a primary fluff-based absorbent layer to form a multi-layer absorbent structure. In these absorbent structures, the primary layer may include superabsorbent polymer granules. With regard to conventionally produced absorbent structures, reference is made to U.S. Pat. Nos. 5,009,650, 5,378,528, 5,128,082, 5,607,414, 5,147,343, 5,149,335, 5,522,810, 5,041,104, 5,176,668, 5,389,181, and 4,596,567, the disclosures of which are hereby incorporated herein by reference.
In recent years, market demand for thinner and more comfortable absorbent articles has increased. Ultra-thin feminine napkins are no longer constructed from loose wood pulp, which tends to give a bulky product, but with roll good-based air-laid absorbent cores in which a roll of preformed absorbent core material is unwound directly onto the absorbent pad-making machine without the defiberization step required for fluff-based products. The roll good-based approach results in a product thinness, which can not be achieved by loose fluff-based technology.
It is known in the art, as described in U.S. Pat. Nos. 3,669,103 and 3,670,731 for example, that carboxylic polyelectrolytes may be cross-linked to create hydrogel-forming materials, now commonly referred to as superabsorbents. These materials are used to enhance the absorbency of disposable absorbent articles. The use of continuous and discontinuous coatings of superabsorbent polymers on various materials is described, for example, in U.S. Pat. Nos. 4,076,673, and 5,071,681 discloses preparation of an air-laid fibrous web and application of a water-insoluble binder to one surface of the web and a water-soluble polymer capable of forming a superabsorbent to the other surface of the web. U.S. Pat. No. 4,444,830 discloses a liquid superabsorbent precursor solution, which is chemically foamed and applied to a base fluffing material, which coated fluffing material is then dried, disintegrated and mechanically worked into a fibrous fluff matrix containing absorbent polymer platelets distributed throughout the matrix.
The above-mentioned approaches for coating fibrous and non-woven materials with superabsorbent-forming compositions cause stiffness of the coated materials due to the inherent brittleness of polyelectrolytes. Humectants such as glycerol may be used to overcome this problem, however, humectants can only plasticize polyelectrolytes in the presence of a substantial amount of water. Maintaining the necessary moisture content is difficult in dry climates. Furthermore, spraying a dilute liquid superabsorbent precursor solution onto an air-laid web may cause deep polymer penetration into the web so that the superabsorbent polymer is not a discrete layer on the surface of the web but a diffuse coating on the fibers of the web.
It has now been surprisingly discovered that it is not necessary nor even desirable to have the superabsorbent polymer material dispersed throughout the absorbent composite for the superabsorbent to work in its intended fashion. Indeed, having the superabsorbent concentrated in a distinct layer, which may be placed (in a disposable absorbent product) away from the source of the liquid insult, provides more efficient utilization of the fibrous material in the absorbent product for initially imbibing and then transporting the fluid. This result is accomplished by foaming a composition containing a water soluble polymer capable of forming a superabsorbent polymer upon drying and curing and applying the foam onto the absorbent web.